I finally organised all of baby's clothes in her dresser and then it hit me, should I have pre-washed them all before she's born?

Yes you'll need to prewash them as they are manufactured in factories. Hence with baby's delicate skin, new clothings may cause your baby's skin to be irritated. Best to prewash ya? Otherwise soak in hot water and dry it, that may do the trick. Easiest way is to throw it into the washing machine with baby detergent.